一、引言在四时四频编码系统中,发端将信源的数字信号按每两比特分组,得到四种不同的信号组合11、10、01、00。采用四个不同的频率f_1、f_2、f_3、f_4对上述组合进行编码,构成四组相应的频率脉冲码f_1f_2f_3f_4、f_2f_4f_1f_3、f_3f_1f_4f_2、f_4f_3f_2f_1。这四组码是非循环正交码,如 I. INTRODUCTION In the four-four-frequency encoding system, the originator divides the digital signal of the source into two bits for each packet, and obtains four different signal combinations 11, 10, 01, 00. The above combination is encoded by four different frequencies f_1, f_2, f_3 and f_4 to form four corresponding frequency pulse codes f_1f_2f_3f_4, f_2f_4f_1f_3, f_3f_1f_4f_2 and f_4f_3f_2f_1. These four codes are acyclic orthogonal codes, such as